Multiple collector mass spectrometers are known with which it is possible to measure the isotopic ratios of different elements. In these devices, the relative positions of the various collectors are usually made adjustable because the spacing between the ion beams of different mass-to-charge ratios at the collectors is dependent on the actual value of the mass-to-charge ratios as well as the difference between them. Also, due to aberrations, the theoretically predicted positions of the beams may not be borne out in practice and adjustments in the collector positions may have to be made.
A device which permits the adjustment of collector spacings is shown in U.S. Pat. No. 4,524,275 "Multiple Collector Mass Spectrometers" by J. S. Cottrell et al. This shows a fixed central aperture with a plurality of movable apertures either side. Another device with movable collectors is shown in U.S. Pat. No. 3,522,428 "Mass Spectrometer having a Plurality of Relatively Movable Collectors" by P. Powers. This device employs a number of positionable collectors sliding on a track.
Another method of varying collector aperture spacing is shown in U.S. Pat. No. 4,595,831 "Multiple Mass Range Triple Collector Spectrometer" by E. A. Hetherington Jr. In this device the collector apertures are on a rotating plate behind which are positioned Faraday cup detectors. The plate has a plurality of aperture clusters at various positions and it can be rotated so that different aperture clusters are presented to the beam. One major disadvantage of this design is that it is only possible to select between a fixed number of aperture spacings, and these cannot easily be altered.
In addition to variable aperture spacing, it is also desirable to have variable aperture size in order to be able to optimise the spectrometer resolution over the range of the different elements to be studied.
In single slit mass spectrometers, many different techniques are known for varying aperture size, see for example GB2146790 "Device for adjusting slit widths in spectrometers", assigned to Finnigan Mat GmbH. In this apparatus the slit jaws are adjusted by a piezoelectric element. A single adjustable slit is also shown in U.S. Pat. No. 3,655,963 "Device for controlling the slit width of adjustable slit electrodes in mass spectrometers" by Brunnee et al. In this device, the slit width is adjusted by means of a heated wire, the length of which varies with temperature.
These known systems have disadvantages--the piezoelectric adjustment mechanism is expensive, and the heated wire mechanism can be unreliable. Also, the use of a mechanical aperture size adjustment technique in combination with adjustable aperture positioning would involve a great increase in complexity and expense, as a separate mechanical linkage passing through the wall of the vacuum housing has to be provided for separately adjusting both the size and position of each aperture. At the high vacuum necessary for the proper operation of isotope-ratio mass spectrometers, this involves much complex and expensive engineering.